Return `CStr` in `goblin::strtab::Strtab` instead of `str`

There is no way to get access to null-terminated C strings that are **not** valid UTF-8, through `goblin::strtab::Strtab`. `Strtab.get()` and `Strtab.get_at()` should return `Option<&CStr>` instead of optional UTF-8 `str`s.

Start by locating goblin::strtab::Strtab and the get() and get_at() methods, then inspect their callers and existing tests. The change is done when both methods expose null-terminated strings that are not valid UTF-8 through Option<&CStr>, with affected callers and tests updated accordingly.

Written by the indexing model from the issue text.